1. Field of the Invention
The present invention relates to a pressure sensor, and in particular, to a pressure sensor having a digital output, a method of fabricating the same and a method of calibrating such a pressure sensor.
2. Description of the Related Art
A pressure sensor senses a pressure in a process or system and widely used in industrial measurement, automatic control, medical application, control of an automotive engine, environmental control, electric appliances or the like. A measuring principle of such a pressure sensor widely known in the art is to use displacement, distortion, magnet-heat conductivity, frequency or the like as a parameter.
Recently, such pressure sensors have been greatly improved in performance compared to cost and it has become possible to mass-produce them with the aid of developed semiconductor and micromachining technologies. In particular, multiplicated sensors formed by integrating plural pressure sensors in a single chip, multi-functionalized sensors formed by integrating sensors of different functions and intelligentized sensors formed by integrating electronics are being developed using the micromachining technologies.
Pressure sensors may be generally classified into mechanical type, electronic type and semiconductor type.
Representative examples of the mechanical type sensors are bourdon tube type sensors, diaphragm type sensors, and bellows type sensors. Among others, the elastic bourdon tube type sensors are most widely used today, in which a principle is employed that if a pressure to be measured is introduced into an open fixed end of a metallic pipe of a circular or planar cross-section, a closed tip end of the pipe is moved.
The most widely used type sensors next to the bourdon tube type sensors are flat plate type sensors and bellows type sensors, wherein the former measures a pressure on the basis of a flexure degree of a circular plate, which is proportional to a pressure difference, and the latter measures a pressure on the basis of a displacement of a bellows caused by a pressure difference between the interior and exterior of the bellows, in which the displacement is proportional to the pressure difference.
Most of the electronic type sensors are basically same with mechanical type ones except that a mechanical displacement is converted into an electric signal. For example, a capacitive type pressure sensor basically uses a method of measuring a displacement between two objects (electrodes) on the basis of the change in electrostatic capacity between the two objects.
Beyond the above-mentioned types, there are piezo-resistive sensors employing a strain gauge, piezo-electric sensors employing an organic or inorganic piezo-electric device, and LVDT inductive coil type sensors. Recently, optical pressure sensors employing an optical fiber or optical path difference have been developed and utilized for the purpose of sensing an ultra-high temperature environment or remote-sensing. Because the piezo-resistive sensors attain superiority over the other types in terms of performance and costs, they are most widely used.
In addition, the semiconductor pressure sensors, of which the practical use is accelerated, are characterized by no creep phenomenon, superiority in linearity, small-size and light-weight, and high resistance to vibration. Moreover, they are better than the mechanical type sensors in terms of sensitivity, reliability and productivity.
A semiconductor sensor comprises a diaphragm for converting a pressure into a distortional stress, and means for converting a power generated in the diaphragm into an electric signal. The diaphragm is formed by chemically etching single crystal silicon. Although the stress generated in the diaphragm may be converted into an electric signal using a change in natural frequency and surface acoustic wave of an oscillator, piezo-resistive and electrostatic capacitive modes are mainly used in converting the stress into an electric signal.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional piezo-resistive pressure sensor. The piezo-resistive pressure sensor comprises a diaphragm 115 formed by etching an n-type semiconductor substrate 110 to a predetermined depth in relation to the bottom surface of the semiconductor substrate. In addition, p-type impurities are diffused at predetermined areas of the top surface of the n-type semiconductor substrate 110 to form p-type impurity regions 114, and the p-type impurity regions 114 are connected with each other by piezo-resistive devices 116.
Electrodes 122 are provided on the p-type impurity regions 114 of the opposite ends through a laminated oxidation film 120 to detect an electric signal produced in response to a change in resistance of the piezo-resistive devices 116.
The above-mentioned piezo-resistive pressure sensor measures a pressure using the change in resistance of the piezo-resistive devices 116 caused by an external pressure. The pressure sensor may be driven through a constant voltage mode or a constant current mode. However, each driving mode requires a compensation circuit capable of coping with a temperature because a piezo-resistive coefficient has a negative temperature characteristic.
FIG. 2 is a cross-sectional view illustrating a structure of a conventional electrostatic capacitive pressure sensor. The electrostatic capacitive pressure sensor includes a diaphragm 215 formed by etching an n-type semiconductor 210 to a predetermined depth in relation to the bottom surface of the semiconductor, a bottom electrode 212 formed by diffusing p-type impurities to a predetermined depth in relation to the top surface of the n-type semiconductor substrate 210, and a top electrode 216 formed using metal on the top surface of the n-type semiconductor substrate to confront with the bottom electrode 212.
The above-mentioned electrostatic capacitive pressure sensor experiences a change in electrostatic capacity between the electrodes 212, 216 facing one another if the distance between the electrodes 212, 216 is changed by a stress according to an external pressure. The stress can be detected by converting the change of electrostatic capacity into an electric signal.
Because no characteristic peculiar to semiconductors is applied to the electrostatic capacitive pressure sensor, the electrostatic capacitive pressure sensor is not necessarily limited to semiconductors. However, single crystal silicon is widely used in fabricating such an electrostatic capacitive pressure sensor because the single crystal silicon is superior as a material for a diaphragm and easy to fine-machine.
Although electrostatic capacitive pressure sensors are superior to piezo-resistive pressure sensors in sensitivity, there is little demand for the electrostatic capacitive pressure sensors because they have a structure that makes the formation of electrodes and connection to an external circuit complicate and they are poor in responsive performance. However, the electrostatic capacitive pressure sensors are very advantageous when they are used in a low-pressure area such as a living body because they are superior in temperature characteristic, small-sized and highly sensitive.
As described above, although a piezo-resistive pressure sensor and an electrostatic capacitive pressure sensor have several advantages in their own ways, the former has a disadvantage in that it is affected by a temperature and current consumption is high and the latter has a disadvantage in that it requires an external circuit for converting an electrostatic capacity into a voltage.
Furthermore, both of the piezo-resistive pressure sensor and the electrostatic capacitive pressure sensor have problems in that they both require a separate analog-digital converter because they employ an analog type interface, and the external circuits thereof are complicated because they require various amplifiers and signal modulators so as to determine an electrostatic capacity or resistance.
Consequently, existing pressure sensors have problems of high costs and difficulty in fabricating them as well as high consumption of power, due to the complexity of peripheral circuits rather than due to the complexity of the sensor structures themselves.